1. Field of the Invention
The invention relates to the use of plasma polymer layer sequences as functional layers in material transport or heat exchanger systems, by means of which these functional surfaces have definitely adjustable wetting behavior. Their use is advantageous in many fields in which material transport or heat exchange systems are used.
2. Discussion of Background Information
The supply and dissipation of heat or material transport is of central importance for many process technical industrial processes. In the transmission of heat, it is advantageous to use phase transitions (condensation, evaporation) since in this way, very high heat flow densities can be achieved. This will be explained in detail in the procedure of condensation.
In the majority of technical condensers, the fluidization of vapor occurs as film condensation. The heat transfer coefficient can be increased by approximately one order of magnitude if the condensation is carried out as dropwise condensation. In order to achieve this, the surfaces on which the condensation takes place should have a definite wetting behavior which is distinguished by very low surface energies or by an edge angle of &gt;60.degree. relative to water.
It is known that the basic prerequisite for the dropwise condensation is the existence of hydrophobic surfaces (low surface energy) in the condenser. The definite production of condenser surfaces of low surface energy requires the use of specific coatings on the metal parts that are necessary for heat transmission reasons.
It is furthermore known that monomolecular layers based on Langmuir-Blodgett films can have low surface energies. The extremely low thickness of these layers and the low number of possibilities for optimizing their mechanical/corrosive properties make them largely unsuitable for use in technical condensers for longevity reasons.
Other layers are also unsuitable if they cannot withstand the mechanical/corrosive stresses. For the mechanical/corrosive resistance of the layer, if a layer thickness is required that is so great that the thermal resistivity connected with it does not, on the whole, lead to any intensification of the heat transmission capacity (possibly even to a worsening in comparison to the initial system), then this layer is also unsuitable.
Materials that adhere to the metallic base material via physical and/or chemical effects and have been used to achieve incomplete wetting for tests of dropwise condensation include organic compounds with hydrophobic groups, various plastics, inorganic substances, special alloys, and--not without contradictions of different results--precious metals as well. Table 1 represents only a few references for this purpose.
TABLE 1 represents the promoting agent, wherein promoter is understood here to mean all substances effectively used to hydrophobize the base material, to bring about dropwise condensation (the essential tasks, no claim as to completeness). The examples from the literature numbered here are taken from the list in Appendix 1. Material/ substance Notes Reference organic 37 different compounds, Blackmann et al. (1) compounds 16 of them tested for 500 hours montan wax Watson et al. (2) montan wax Tanner et al. (3) oleic acid Finnicum & Westwater (4) silicone oil Mingdao and Jiliang (5) diff. promoters, overview Kast (6) of different results polymers fluoroacrylic, parylene, Marto et al. (7) Emralon 333, fluoroepoxy polymerized Marto et al. (8) triazine dithiol films on Al/Mg tubes Mori et al. (9) PVC layers PTFE layer Haraguchi et al. (10) PTFE wetting analysis Utaka et al. (11) Boyes & Ponter (12) inorganic sulfides and Se compounds Erb & Thelen (13) compounds galvanic dispersion layer, esp. Cluistra et al. (14) with PTFE dispersion alloys surface alloy, CU with Zhang et al. (15) Cr, Fe, Al, Bi, Sb, Sn, Se, In Ion implantation N, He, Zhao et al. (16) Ar, H Ion implantation Cr, N Zhao et al. (17) in Cu precious metals Gold, according to this, Bernett & Zisman (18) pure gold surfaces can and Smith (19) be completely wet Gold gilded with paraffin- Wilkins et al. (20) thio-silane, and mercaptan, vacuum coated gold coatings, partially chemically treated silver, dep. via electroplating Tanasawa & Westwater (21) Woodruff & Westwater (22) Barthau (23) O'Neill & Westwater (24)
In all of the tests of dropwise condensation, in addition to the clarification of purely heat technical questions, the test of conceivable promoters forms the basis of the achievement of dropwise condensation and constitutes the main focus of the research.
Out of all of the research results that indisputably demonstrate possibilities for achieving dropwise condensation, until now, no methodology has come into use in the practical construction of condensers. This is brought about by the still insufficient proofs as to the service life of promoter layers and the resulting lack of certainty of a long term maintenance of dropwise condensation.
It is now the object of the invention to find functional layers that can be deposited by means of conventional coating methods on the functional surfaces of the corresponding apparatuses and devices for these material transport or heat exchanger systems, e.g. condensers, whose use will eliminate the disadvantages of the prior art.
It is consequently the object of the invention to find functional layers of the type mentioned, which can be used in all material transport or heat exchanger systems, e.g. also or even especially in condensers, which in the event that the functional layers are used on condensers, force the dropwise condensation and can advantageously be used in the systems and apparatus construction.
According to the invention, this object is attained through the use of a plasma polymer layer sequence as a functional layer in material transport or heat exchanger systems.
In these material transport or heat exchanger systems with a definitely adjustable wetting behavior, the functional layers that are deposited on the base material of the functional surfaces of apparatuses for heat and material transfer, plasma-modified carbon--hydrogen layers (a-C:H layers or DLC (diamond-like carbon) layers). These plasma-modified carbon--hydrogen layers are comprised of a plasma polymer layer sequence as a hard material layer with a top layer as a functional layer as well as possibly an intermediary layer and a gradient layer, wherein for adjusting a definite wetting behavior, in addition to the base elements of carbon and hydrogen, the top layer contains other non-metallic or metalloid elements of the 1st, 3rd, 4th, 5th, 6th, and/or 7th main group of the periodic table of the elements (preferably 1 at % to 70 at % in relation to the carbon content). These layers can be produced using conventional PVD or CVD processes, e.g. a plasma-enhanced chemical vapor deposition (PECVD) process. These layers are known per se and are described in DE 44 17 235, even with regard to manufacturing possibilities.
It has surprisingly turned out that in particular assembly regions of the top layer, these layers can be used to excellent effect as functional layers in or on
plate heat exchangers (composition of the top layer: 1-40 at % F, Si, and/or O, 60-99 at % C, with a remainder of hydrogen up to max. of 30 at %) PA1 tubular or tubular bundle heat exchangers (composition of the top layer: 1-40 at % F, Si, and/or O, 60-99 at % C, with a remainder of hydrogen up to max. of 30 at %) PA1 filling bodies in filling body columns (composition of the top layer: 1-40 at % N,O and/or B, 60-99 at % C, with a remainder of hydrogen up to max. of 30 at %) PA1 condensers for forcing dropwise condensation (composition of the top layer: 1-40 at % F, Si, and/or O, 60-99 at % C, with a remainder of hydrogen up to max. of 30 at %) PA1 heat/material exchanging walls and apparatus walls lead to a deliberate influencing of the adhesion mechanisms in such a way that the soiling inclination, the depositing inclination, and the caking inclination are advantageously counteracted (composition of the top layer: 1-40 at % F, Si, and/or O, 60-99 at % C, with a remainder of hydrogen up to max. of 30 at %).
The inadequacies of the prior art can be bypassed through the use of these specially adapted plasma polymer layers with adjustable, defined wetting behavior since in addition to the required hydrophobic property, they simultaneously have an extremely high wear resistance and service life and this is the case at layer thicknesses that have no significant heat resistivity.
The layers for use as a functional layer utilized in heat exchanger systems are plasma-modified amorphous hydrocarbon layers (a-C:H layers or DLC layers (diamond-like carbon)), which are usually deposited in a vacuum by means of a high frequency plasma process. But other processes, such as the pulse plasma technique, the middle frequency excitation, twin Mac process, diode sputtering, or the hollow cathode process, and other processes can also be used. Amorphous hydrocarbon layers can be deposited on a wide variety of substrates by introducing acetylene into a high-energy glow discharge (plasma), which is maintained, for example, by means of a high frequency coupling in the MHz range. These layers are deposited at a rate of a few mm/h. The chemical structure of the a-C:H layers is similar to that of highly cross-linked polymers, only the degree of cross-linkage is much higher. The degree of cross-linkage and therefore the layer properties can be influenced within a certain scope by means of a suitable carrying out of the process. Chemically speaking, purely amorphous hydrocarbon layers are merely comprised of carbon and hydrogen, wherein approx. 70-95 atom percent of carbon (97 wt. %) and 5-30 atom percent hydrogen is typical in the layers.
The high cross-linkage is the reason for the chemical stability of the layers as well as their resistance to the indiffusion of relative components, which could lead to a destruction of the base material (substrate). Fundamentally, the utility of the a-C:H layer material as a corrosion protection layer has been demonstrated. The a-C:H layer system is chemically resistant to oxidizing and non-oxidizing acids such as HNO.sub.3 and HCl, lyes such as NaOH, and commercially available solvents.
Amorphous hydrocarbons deposited in a vacuum have been known as layering systems since approximately 1975. Taking into account suitable substrates and their pre-cleaning, it is basically no problem to coat both conducting and insulating surfaces. Layer thicknesses in the region of a few mm are deposited, wherein the growth rate of the layers is in the neighborhood of a few mm per hour. Hydrocarbon layers, commonly enriched with a metal component, are most widely used in the tribological region, where they are used to reduce coefficients of wear and friction. Improvements in wear behavior by a factor of 100 are achieved, in comparison with the wear behavior of steel.
Building on the basis of a-C:H layers (DLC layers), through the deliberate incorporation of other elements into the highly cross-linked hydrocarbon matrix, the wetting behavior can be deliberately influenced in relation to a wide variety of solvents, in particular, though, in relation to water. The incorporation of fluorine into the layers succeeds in producing a surface which, in its wetting behavior, corresponds to that of Teflon.RTM., wherein, however, the very favorable wear behavior of the layers is essentially maintained. Similar effects can be achieved as well with the modification element silicon. The adjustability of the surface energy is not absolutely visible, but can on the contrary be adjusted deliberately via the fluorine or silicon content of the layers. This property of low surface energy is used to force the dropwise condensation in heat exchanger systems.
While fluorine and silicon markedly reduce the surface energy and consequently lead to a lower wetting by water, the opposite effect can be achieved by incorporating elements such as nitrogen, boron, or oxygen. These elements markedly increase the surface energy of a coated surface and lead to a clearly improved wetting by fluid media. This effect is utilized when the layers are used in systems for material transmission and material transport.
In addition to the modification of the overall surface energy, the corresponding combination of modification elements is also successful in the deliberate influencing of the polar and dispersed surface energy proportions, which add to the overall surface energy. Consequently, systems can be optimally adapted for their particular use.
The use of layers is meaningful not only in the field of condensation and material transport, but also in the field of heat transfer when boiling. Due to the thermal stability of the layers, there is a limit here that lies at the maximal temperature load of the layers of 450.degree. C.
Another effect directly connected to the surface energy is that a clear reduction of the surface energy can also prevent a soiling of the surfaces in heat exchanger systems. The lower surface energy leads to a lower adhesion of dirt particles (e.g. the formation of sediment) and therefore to a greater ease in cleaning heat exchanger systems of this kind. Furthermore, through the use of the functional layers explained here, deposits and cakings of granular materials can also be prevented or reduced because a reduced surface energy counteracts the adhesion capability of particles to the wall [25].
The advantages of the functional layers are also a result of the fact that by means of their use in material transport and heat exchanger systems of this kind, properties such as an adjustable wetting behavior, wear protection functions, prevention of a corrosive action and/or of the topography-obtaining action, even in combination with one another, can be made useful for heat exchanger systems.
The invention will be explained in detail in conjunction with the following exemplary embodiments.